Capacitor for sensing contaminated oil

ABSTRACT

Apparatus to determine the dielectric characteristics of a fluid dielectric material including a rigid mounting with a planar mounting surface, a base electrode on a stem secured to the mounting surface and having a broad disc-like portion at one end of the stem and defining a flat electrode face, a ring electrode concentric with the flat face of the base electrode and having an annular wafer-like shape and a mounting projection extending from the outer edge of the wafer-like ring electrode to the planar surface of the rigid mounting and affixed thereto, and a solid insulating media between the base and ring electrodes and forming the remainder of a container wall adjacent the electrodes, the insulating media being formed of a stable low dielectric material which is substantially insensitive to temperature changes.

United States Patent 1 Stoakes Apr. 8, 1975 1 CAPACITOR FOR SENSINGCONTAMINATED 01L [76] Inventor: Donald S. stoake s, 2506 Grand Ave.

South, Minneapolis, Minn. 55405 [22] Filed: Jan. 8, 1973 [21] Appl. No.:321,912

[52] 11.8. C1 317/249 R; 317/246; 317/247; 317/248; 324/61 R [51] Int.Cl. ..1'10lg 5/16; H0lg 7/00 [58] Field of Search 317/246, 247, 248, 249R; 324/61 R. 60

[56] References Cited UNITED STATES PATENTS 1,960,168 5/1934 Schoenberg317/61 R 3,182,255 5/1965 Hopkins 324/61 R 3,192,455 6/1965 Bergeson317/247 3,460,011 5/1969 Kudlcc 317/247 3,746,974 7/1973 Stoakes 317/246UX Primary E.ranzinerE. A. Goldberg Attorney, Agent, or Firm-H. DalePalmatier ABSTRACT Apparatus to determine the dielectric characteristicsof a fluid dielectric material including a rigid mounting with a planarmounting surface, a base electrode on a stem secured to the mountingsurface and having a broad disc-like portion at one end of the stem anddefining a flat electrode face, a ring electrode concentric with theflat face of the base electrode and having an annular wafer-like shapeand a mounting projection extending from the outer edge of thewafer-like ring electrode to the planar surface of the rigid mountingand affixed thereto, and a solid insulating media between the base andring electrodes and forming the remainder of a container wall adjacentthe electrodes, the insulating media being formed of a stable lowdielectric material which is substantially insensitive to temperaturechanges.

16 Claims, 7 Drawing Figures CAPACITOR FOR SENSING CONTAMINATED OILBACKGROUND OF THE INVENTION Reference is made to U.S. Pat. No. 3,746,974entitled Oil Permittivity Sensor.

When the dielectric characteristics of a material can be determined,significant conclusions can be reached about the material itself. Lowdielectric materials are essentially non-conductive of electricalcurrent and include many liquid materials such as various oils andpetroleum products including lubricating oil, hydraulic fluids, keroseneand gasoline, and other liquids such as alcohol, molten plastics such aspolyethylene, ABC, styrene, and other liquid materials such as moltenglass, printing ink, molten rubber, etc. Low dielectric gases includemany common gaseous materials such as methane, natural gas, automobileand diesel exhaust gases, combustion flue gases, air, Freon, and thesulfites and sulfates.

It has been found that as many such dielectric materials are used,impurities and contaminants will be picked up in the material. Bysensing and measuring the dielectric characteristics of these materials,the presence and the relative quantity of such impurities orcontaminants can be determined. Of course, the dielectriccharacteristics of samples being measured will be compared topredetermined normal characteristics so that proper conclusions can bedrawn as to the nature of the samples being tested.

It should further be noted that high dielectric materials, which arerelatively conductive, exhibit the same general characteristics suchthat the dielectric characteristics of the material will vary with thepurity or impurity ofthe material. To determine the dielectriccharacteristics of high dielectric material will permit significantconclusions as to the nature of the material and the contaminants whichmay be contained therein.

IMPORTANT CONSIDERATIONS RELEVANT TO THE PRESENT INVENTION Thedielectric constants for various materials vary extremely widely. Forcertain materials, the dielectric constant is l, and for other materialsthe dielectric constant is as high as 12,000. For any particularpercentage change in the dielectric constant, the actual change in thedielectric constant of a low dielectric material will be verysignificantly smaller than the actual change in the dielectric constantof a high dielectric material.

This concept becomes extremely significant when measuring dielectricchanges in low dielectric liquids such as lubricating and hydraulicoils. It has been found through correlated laboratory tests that often a5 percent change in a lubricating oils dielectric constant representsthe entire range of measurement, from a new and unused oil to an oilcontaining such oxides and contaminants that the oil is unfit forfurther use; and, similarly, in the case of hydraulic oils, frequently a2.5 percent change in the dielectric constant is the full range ofchange from new and unused oil to an oil so contaminated that it isunfit for further use as a hydraulic oil. The dielectric constants ofhydraulic and lubricating oils are 2.0 and 2.2, respectively, in new andunused condition, and therefore the changes in the actual dielectricconstants of 0.05 and 0.1 1, respectively, represent the totaldielectric measurement ranges of these oils. These relationshipsemphasize that the sensor for determining the dielectric characteristicsof the fluid must be extremely sensitive and stable so that results canbe relied upon.

Furthermore, temperatures of the oil may vary widely in test conditions,particularly where lubricating oil in an engine isbeing monitored as itrecirculates. The minute changes in dielectric characteristics must bemeasured even though ambient temperatures of the air at the exterior ofthe engine may vary F., and temperatures of the dielectric materialbeing sampled will vary as much as 300F.

By contrast, to the hydraulic oil, a 2.5 percent change in thedielectric constant of water is an increment approximately 39 timeslarger than the increment of change of the hydraulic oil. It isimportant that the sensor be able to detect these large changes indielectric characteristics as well as the extremely minute changes.

Whereas in the prior art, it is asserted that the dielectric constant oflubricating oil decreses 400 percent from 100F. to 200F., thatassertion, which has been popularly accepted, is false. The dielectricconstant of oil does not change significantly with the temperature ofthe oil, within the range of 15F. to 350F.

BRIEF SUMMARY OF THE INVENTION The present invention is a sensor whichhas maximum sensitivity to low dielectric material and maximum stabilitythroughout wide changes in temperatures.

The sensor has two electrodes insulated from each other to form acapacitor. One electrode, which may be referred to as the baseelectrode, has a flat horizontal electrode surface, the edge of whichhas a certain shape such as a circle; and the base electrode isordinarily ungrounded for applying a signal or voltage to it. Theungrounded base electrode is formed of metal, such as aluminum orberyllium copper, and extends downwardly from the electrode surface to ahorizontal reference plane lying parallel to the electrode surface. Thebase electrode is supported at said reference plane.

The second or ring electrode is ordinarily grounded and is formed of thesame material as the ungrounded base electrode. The ring electrode issupported at the same reference plane from which the undergrounded baseelectrode is supported. The grounded ring electrode has an inner annularelectrode surface of uniform width lying normal to the spaced from theflat electrode surface of the base electrode, the flat and annularelectrode surfaces having similar surface areas. The annular surface ofthe ring electrode has the same shape and orientation as the edge of theflat electrode surface and is uniformly spaced from said edge.Accordingly, the annular electrode surface lies normal to the referenceplane and the end edges of the annular electrode surface lie parallel tothe reference plane.

The ring electrode extends horizontally outwardly in all directions fromthe annular surface and has a depending leg portion spaced outwardlyfrom the ungrounded base electrode and extending down to the referenceplane at which the ring electrode is supported.

A rigid insulator of a material with stable characteristics, such asmica filled fluorocarbon, surrounds the sides of the base electrode andunderlies the ring electrode so as to leave both the flat and annularelectrode surfaces exposed and confronting each other and cooperativelydefining a sample chamber or space to confine a quantity of thedielectric material, the characteristics of which are to be determined.

In one form, the sample chamber is increased in height, above the ringelectrode by an impervious wall to materially increase the depth of thesample of dielectric liquid.

In another form, the sensor may be wholly confined in a non-metallicflow line as for lubricating oil in an engine, so that the entirequantity of oil in the lubrication system is continually being sensed asto its characteristics.

Another aspect of our invention is a monitoring apparatus tocontinuously sense and determine the dielectric characteristics of adielectric material. A capacitance bridge incorporates a pair ofcapacitance sensors previously described, wherein one of the sensorsmonitors and senses the continuously change test sample of lowdielectric material as the material circulates and recirculates duringuse. The second sensor continuously monitors and senses thecharacteristics of an unused sample of such material to provide a normagainst which a comparison is made in the bridge circuit.

Although the second sensor is virtually unaffected by the temperature ofthe sample, the unused sample is maintained at the same temperature asthe continuously changing test sample. Furthermore, the utilization ofthe second sensor readily facilitates initial balancing of thecapacitive bridge circuit and eliminates other capacitors which may beseverely affected by temperature.

Because the flat and annular electrode surfaces are oriented normal toeach other, the sensor has a minimum of static capacity. Supplying oilbetween the electrodes to constitute the dielectric will have themaximum effect upon the capacity of the sensor; and there will be amaximum change in the dielectric characteristics between new and usedoil. Maximum capability is thereby achieved.

The electrode surfaces confront each other obliquely and the electrodesare closest to each other only along spaced and juxtaposed edges.-

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a section view taken on anupright plane through a sensor embodying the present invention.

FIG. 2 is a schematic circuit diagram of a preferred circuit for use inconnection with the sensor of FIG. 1.

FIG. 3 is a section view taken on an upright plane through a modifiedform of sensor.

FIG. 4 is a transverse section view through another form of sensorembodied in an apparatus for cont nually sensoring and mointoring thedielectric charac eristics of a liquid such as the lubricating oil of aninternal combustion engine.

FIG. 5 is a detail section view taken approximately at 5-5 of FIG. 4.

FIG. 6 is a bottom plan view of the apparatus of FIG. 5 and being partlybroken away and shown in section along a broken line 6-6 as illustratedin FIG. 5.

FIG. 7 is a schematic circuit diagram of a preferred form of bridgecircuit for use in connection with the apparatus of FIGS. 4 6.

DETAILED DESCRIPTION OF THE INVENTION The form of sensor illustrated inFIG. 1 embodies all of the essentialcharacteristics of the sensor fordetermining the dielectric characteristics of a fluid. The sensor isindicated in general by numeral 50 and has a base electrode 51 and aring electrode 52, both of which are formed of the identical materialwhich is preferably a heat treated beryllium copper compound with a lowcoefficient of linear expansion, but other metals, including aluminum,are acceptable. Both the base electrode and ring electrode are supportedby and anchored securely to a rigid mounting panel 53 which has verystable characteristics so as to hold the electrode 51 and 52 instationary relation with respect to each other; and the mounting panel53 may be an epoxy resin sheet or a fiberglass panel. The upper surface53.1 of the mounting panel defines a reference plane from which both theelectrodes will expand or contract with temperature variations.

The sensor 50 also includes an insulator 54 between all portions of theelectrodes 51, 52, with the exception of the active capacitor facesthereof. The insulator 54 is preferably a low dielectric material withstable dielectric characteristics and expansion and contractioncharacteristics over wide temperature changes, and a typical materialmay be mica filled fluorocarbon.

More specifically, with respect to the sensor 50, it will be noted thatthe base electrode 51 has a substantially flat electrode face 51.1 whichlies substantially parallel to the top surface or reference plane 53.1of the mounting. In this particular form, the electrode face 51.1 iscircular in shape and has a circular peripheral edge, but the face mighthave some other regular or irregular shape. The principal portion of theelectrode 51, is substantially disc-shaped, and the peripheral side 51.2of the disc-shaped portion of the electrode is substantially cylindricalin shape. This side 51.2 may, in some forms of sensor, be beveled ortapered to be somewhat conical in shape in a downwardly convergentdirection.

The base electrode 51 has a mounting stem 51.3 of substantially reducedthickness as compared to the diameter of the circular electrode face.The stem 51.3 is threaded and is threadably mounted in an internallythreaded socket sleeve 55 which is carried on a rigid with the mountingplate 53. In this form, a sealing O- ring or gasket 56 seals the upperportion of stem 51.3 to the insulation block 54 to prevent any migrationof the dielectric material being tested.

The ring electrode 52 has a flat wafer-shaped portion to the uppersurface or reference plane 53.1 of the mounting panel. The upper andlower surfaces of the substantially annular wafer-shaped portion 52.1 ofthe ring electrode are preferably smooth and parallel to each other. Thering electrode 52 also has a mounting projection in the form of acylindrical wall 52.2 extending from the annular wafer-shaped portion52.1 of the ring electrode to the upper surface or reference plane 53.1of the mounting panel. The cylindrical wall 52.2 is formed integrally ofand in one piece with the annular wafer-shaped portion of the ringelectrode, and if desired, the projection portion 52.2 could be in theform of spaced legs around the periphery of the ring electrode insteadof being a continuous cylindrical wall. It will be recognized that thelower edge of the cylindrical mounting projection 52.2 bears against themounting panel 53, and is secured thereto by screws 56 which are screwedinto tapped apertures'in the ring electrode 52.

The ring electrode 52 defines an annular electrode face 52.3 which, inthe form illustrated, is cylindrically shaped and of the same diameteras the flat electrode face 51.1; and the annular electrode face 52.3 ispositioned in spaced and coaxial relation to the flat electrode face51.1. In the event that the flat electrode face 51.1 has a shape otherthan circular, then the annular electrode face 52.3 would also have thatsimilar shape, identical to the shape of the peripheral edge of the flatelectrode face 51.1, and the annular electrode face 52.3 would also havethe same size and orientation as the flat electrode face 51.1.

The annular electrode face 52.3 must be oriented substantiallyperpendicular to the flat electrode face 51.1 and to the top surface orreference plane 53.1 of the mounting panel.

The relative spacing between the peripheral edge of the flat electrodeface 51.1 and the lower peripheral edge of the annular electrode face52.3 is fixed and controlled by the electrodes 51 and 52 themselves intheir relationship with the mounting panel 53, and the insulation 54,which is considerably weaker in its strength characteristics than themetal of the electrodes, does not have any appreciable effect on thephysical relationship maintained between the two electrodes.

The flat and annular electrode faces 51.1 and 52.3 preferably haveapproximately the same areas, but the annular electrode face 52.3 mayhave an area somewhat less than the area of the flat electrode face51.1. It is highly preferable that the width of the annular wafer-shapedportion 52.1 of the ring electrode 52, as measured across the lowersurface in a direction outwardly from the annular electrode face 52.3 tothe cylindrical mounting projection 52.2 be the same as the distancefrom the center of the flat electrode face 51.1 to the peripheral edgethereof, which in this situation wherein the flat electrode face 51.1 iscircular, is equal to onehalf the diameter thereof.

It is important that there is no space between the periphery of baseelectrode 51 and the insulation 54, and further that the insulationengage and seal against the lower surface of the ring electrode 52. Theinsulation 54 is shaped to have an opening receiving the mounting stem51.3 and to define a wall 54.1 coextensive with the annular electrodeface 52.3 and extending between the annular electrode face and theperipheral edge of the flat electrode face 51.1. The insulation 54 isrelatively yieldable as compared to the substantially rigid electrodes51, 52 which have rather high strength characteristics in relation tothe low strength characteristics of the insulation. It will be notedthat the coextensive annular electrode face 52.3 and the wall 54.1 ofthe insulation cooperatively define the periphery of a container orchamber, the bottom of which is formed by the flat electrode face 51.1of the base electrode. The periphery of this container is extendedupwardly by an overlying plastic shroud 57 which seals downwardlyagainst the ring electrode 52 .and has a central opening coextensivewith the annular electrode face for the purpose of increasing the depthof the chamber or compartment which will confine the sample of fluidmaterial being tested.

In one embodiment of the sensor 50, the diameters of the flat electrodeface 51.1 and the cylindrical annular electrode face 52.3 may be 0.5inches. The width or height of the cylindrical annular electrode face52.3

will be approximately 0.125 inches. The internal diameter of thecylindrical mounting projection 52.2 of the ring electrode 52 will be1.0 inches; and the spacing between the edges of the electrodes isuniformly 0.044

inches around the entire periphery thereof.

The sensor 50 is peculiarly adapted to be selfcompensating totemperature variations as to resist any changes of inherent capacity byvirtue of the physical relationships or changes of physicalrelationships of the parts of the sensor itself. The principal physicalcharacteristics which control capacity are the surface areas of the flatelectrode face 51.1 and the annular electrode face 52.3; and the averagedistance between the capacitor plates or electrode faces, and, as in thepresent situation wherein the electrode faces are circular andcylindrical, the average distance between these electrode faces will befrom a point on the annular electrode face midway along the length of itto a point on the flat electrode face located a distance inwardly fromthe annular edge thereof equal to one-third the diameter. As temperaturechanges, all portions of the sensor 50, and particularly all portions ofthe electrodes will change to the same temperature. The diameter andarea of the circular flat electrode face 51.1 will enlarge withincreased temperature; the diameter of the cylindrical annular electrodeface 52.3 will enlarge identically with the enlargement of the diameterof the flat electrode face because the width of the annular wafer-shapedportion 52.1 of the ring electrode, measured at the lower surface of thewafer-shaped portion 52.1, is the same as half the diameter of thedisc-shaped portion of the electrode 51. The width or height of theannular electrode face 52.3 will also enlarge with increased temperatureto the extent that the enlarged area of the annular electrode face 52.3will remain the same as the enlarged area of the flat electrode face51.1. i

The height of the cylindrical mounting projection or wall 52.2 of ringelectrode 52 will also enlarge with an increase in temperature, and thedistance from the top surface or reference plane 53.1 of the mountingpanel to the annular wafer portion 52.1 of the ring electrode willaccordingly increase. In a similar manner, the length of the baseelectrode 51, from the top surface or reference plane 53.1 of themounting panel to the electrode face 51.1 will enlarge with an increasein temperature. As a result, the location of the flat electrode face51.1 relative to the location of the cylindrical electrode face 52.3will change slightly, but only to the extent as to proportionatelyoffset the change in the electrode surface area of the sensor.

With respect to the capacity of the sensor, the capacity is determinedby the basic formula:

where C,, is the active sensor capacity, K is the dielectric of theliquid material being measured, A is the area of either of the electrodesurfaces if the areas thereof are equal, otherwise A is the area of thesmaller of the electrode surfaces, k is a constant which varies withunits of measure, and d is the average distance between the areas of theelectrode faces exposed to the dielectric material under measure.

Whereas the area of the annular electrode face 52.3 will be at least assmall as the area of the flat electrode face, the area (A) will be thearea of the annular electrode face 52.3 which may be expressed:

where D is the diameter of the ring electrode face 52.3 and T is thelength of the annular electrode face 52.3 in direction perpendicular tothe reference plane 53.1.

The average distance (d between the flat and annular faces of the baseand ring electrodes is measured between a point midway the height of thering electrode face 52.3 and a point on the flat electrode face 51.1located a distance inwardly from the periphery thereof equalingone-third the diameter. The averagedistance (d) may be expressedtherefore as follows:

changes in temperature of the sensor does not produce any discerniblechange in the shunt capacity as the temperature is changed. The shuntcapacity (C,.) is proportional to the fraction A/d wherein A is the areaof the smaller of the inactive surfaces of the base and ring electrodes,and in this situation, the peripheral areas of the base electrode; andwherein d is the average distance between the inactive electrode areasof the two electrodes. This average distance, as relates to the inactivesurface areas is the distance from the point on the ring electrode wherethe inner periphery of the cylindrical mounting projection or wall 52.2meets with the flat lower surface of the wafer portion 52.1 thereof, andto a point on the base electrode half way along the cylindrical sidesurface 51.2 thereof. It can be determined that temperature changes ofas much as 300F. produce no change in the shunt capacities (C in thissensor 50.

The sensor 50 may be used to determine the dielectric characteristics ofhigh dielectric material as well as low dielectric material. In theevent that a high dielectric liquid is to be tested, one of theelectrode surfaces will be coated with an insulating material such as athin film of Teflon. The sensor 50 may then be employed in the same wayas has been described in connection with determining the dielectriccharacteristics of low dielectric material.

The circuit illustrated in FIG. 2 provides a capacitance bridge wherebythe change in capacitance in the sensor 50 may be detected and measured.in the circuit, a mercury type battery 60 of five to twelve volts hasone side connected to ground 65 and the other side connected to anon-off single pole switch 62 through which power may be supplied intothe oscillator 61. The oscillator must be fairly stable at a fixedfrequency between 2.5 and 10.0 MHz, and also provides a constant 5.0 to10.0 VAC output under no load conditions. The capacitor 63, as well asthe capacitors 79 and 80, is a 0.02 uF, 16 VDC ceramic disc typecondensor. Capacitor 63 couples the output of oscillator 61 into themeasuring output at point 64 and provides DC isolation between point 64and ground 65. The coil 67, as well as coils 76 and 77, is a moldedpowdered iron core, 2.5

mH choke providing low resistance path for DC measuring currents whileoffering high impedance to AC current at the oscillator frequency. Coil67 is connected between point 64 and ground 65. The coil 67 may bereplaced by a 500 to 1,50. ohm resistor with only a minimum effect uponcircuit operation. The coils 68 and 71 may be molded, fixed or variableinductance coils whose value depend primarily upon the capacitance valueof sensor 50 and of capacitor 72. If the sensor is fixed in the relativepositioning of the base electrode and ring electrode, the coil 68 willbe a variable inductance coil, in order to provide for initial circuittuning; and if the sensor 50 is slightly variable whereby the baseelectrode may be adjusted slightly with respect to the ring electrode,the coil 68 may be a fixed inductance coil. Coil 68 and sensor 50 form aseries resonant circuit condition which produces a maximum AC voltage inproportion to the impedance value of sensor 50 between the ground andthe midpoint 69 between coil 68 and sensor 50. it will be recognizedthat the ring electrode is connected to ground 65 and the base electrodeis connected to the point 69 in the circuit.

The capacity of the capacitor 72 must be equal in value with thecapacity of sensor 50 with a pure state dielectric material contained inthe sensor, and if the sensor is fixed, the capacitor 72 is fixed, andif the sensor 50 is adjustable or variable, then the capacitor 72 willbe variable. The capacitor 72 is connected at one side to ground 65 andat the other side through a circuit connection to the coil 71 whichforms a resonant circuit condition with the capacitor 72 which producesa maximum AC voltage, in proportion to the impedance value of capacitor37, between ground 65 and point which is midway between the capacitor 72and coil 71.

A pair of diodes 73 and 74 are respectively connected to points 70 and69 to provide a low impedance path in one direction only for AC electroncurrent flow from point 70, through diode 47 and through capacitor 75and diode 48 to the point 69. Capacitor 75 which is connected in serieswith the diodes 73 and 74 is of the same value and type as capacitor 63and provides a low impedance path for AC current and a DC blockingaction for DC voltages rectified by diodes 47 and 48.

Coils 76 and 77 are respectively connected to the midpoints of thecircuits between diode 74 and condensor 75 and diode 73 and condenser75, and the coils 76 resistor 81 which is a wire wound resistor and hasa center tap connected to one side of a galvanometer 78,

the other side of which is connected directly to the ground. Thecapacitors 79 and 80 are respectively connected to opposite ends of thewire wound resistor 81 and to ground 65.

The resistor 81 is a low wattage type, between 50 anad 2,000 ohms, whosepurpose can be either to provide a zeroing adjustment or a measuringscale for DC electron currents flowing in opposite directions betweenpoints 70 to ground 65 and between ground 65 to circuit point 69 throughmeter 78. The meter 78 is a 25 to 200 microamp DC galvanometer with azero center or offset Zero scale which may function either as ameasuring scale or as an indicator and thereby display any differencesin DC currents flowing through it in opposite directions.

To calibrate the circuit of FIG. 2 for a particular range of liquiddielectric material being tested in sensor 50, the pure state of theliquid dielectric material, representing a midpoint in the measurementrange, is placed in the sensor 50, filling it to the top. Next a vacuumtube voltmeter, set to O to 50 VAC scale, is con-' nected between points66 and ground 65 and switch 62 is shorted across with a jumper wire.Resistor 81 is also shorted across with a jumper wire between theunderground ends of capacitors 79 and 80. While observing the vacuumtube voltmeter, either the sensor 50 or the inductor 68, whichever isvariable, is adjusted until the vacuum tube voltmeter reading is anabsolute minimum AC voltage. Then, while observing both the vacuum tubevoltmeter and meter 78, either the capacitor 72 or inductor 71,whichever is the variable, is adjusted until the vacuum tube voltmeterreads a still further absolute minimum AC voltage, andsimultaneously,the meter 78 pointer is directly on its zero indication point.Thereafter, remove the vacuum tube voltmeter connections from point 66and ground 65 and remove both jumper wires from across switch 62 andresistor 81.

When the pure state or unused dielectric material is removed from thesensor 50, the circuit completely cali brated and ready for use.

In one example of operation, the meter 78 of FIG. 2 may be considered asa measuring scale. A sample of low dielectric liquid material in itspure and unused state is placed in the sensor 50 as in the previouscalibration process and switch 62 is closed to energize the oscillator.Any deviation of the meter 78 pointer away from zero is noted andadjusted to an exact zero by rotating the arm of the adjustment resistoror potentiometer 81. The reference standard of-pure and unuseddielectric material is then removed from the sensor 50 which is wipedclean. Thereafter, and without further adjusting anything in thecircuit, another sample of the same type and brand of liquid, but used,will be placed in the sensor 50.

If the sample then in the sensor 50 contains a contaminant notpreviously contained in the reference sample, the meter 78 will indicatea positive deviation in direct proportion to the percentage change inthe amount of contaminant in the liquid test sample. Likewise, if thesecond test sample had contained less contaminants than theoriginal'reference sample, the meter would indicate a negativedeviation.

The meter scale may be divided into increments of measure representingpercent, dielectric constant, or mere numbers for convenience ofdeviation measure,- ments. Of course, the meter may be supplemented orreplaced by a chart recorder to record the readout.

In the event lubricating oil of an engine is being sampled, the normalmeter deviation will be a positive measurement, indicating the normalbuildup of oxidized particles. However, if there is a negative deviationon the meter, then it will be determined that there is some othercontaminant being found in the oil which might be a small quantity offuel.

In summary, with respect to the sensor 50 and the bridge circuitillustrated in FIG. 2, the dielectric characteristics of a liquidmaterial can be determined by sensing and measuring the material in itspure and un used state and subsequently sampling the same material afterit has been used for a period of time. The nature of the change in thedielectric characteristic can permit the conclusion of the nature of thematerial for performing the function it is desired to perform. Thesensor 50 is insensitive to temperature changes, even up to atemperature change of 300F. It is important that the remainder of thecomponents in the circuit of FIG. 2 be maintained at fairly constanttemperature because the value of the other circuit components such asthe variable capacitor 72 may change.

In FIG. 3, another form of sensor 50' is illustrated. This sensor has adisc-shaped base electrode 51' and a ring electrode 52. A quantity ofstable insulating material 54 confines the sides of base electrode 51and separates the base electrode at its sides from the ring electrode52.

In this form illustrated in FIG. 3, the peripheral sidewall of the baseelectrode 51' is tapered so as to be somewhat conically shaped,converging in a downward direction. The base electrode 51 is secured asby adhesive or mechanical means to the top surface or reference plane ofa rigid mounting panel 53'. Although this form of sensor has no stem onthe base electrode, both the base electrode and the ring electrode aresupported by and secured to the top surface or reference plane of themounting panel. The ring electrode has all the characteristics of thatdescribed in connection with the ring electrode of FIG. 1 with theexception that the cylindrical mounting projection of ring electrode 52'and extending from the annular wafer-shaped portion thereof to themounting panel 53 is somewhat shorter than the corresponding cylindricalmounting projection of sensor 50in FIG. 1.

The beveled or conically tapered side of the base electrode 51 leavesonly a rather sharp edge at the periphery of the flat electrode facethereof so that the stray capacity between the base electrode 51' andthe ring electrode 52' is considerably reduced. The conically taperedside of the base electrode does not squarely confront any of the lowersurfaces of the ring electrode 52', but is disposed at extremely sharpangles with respect to all of the lower surfaces of the ring electrode52'-so as to reduce the stray capacity.

The form of the invention illustrated in FIGS. 4 7 includes a dualsensor unit indicated in general by numeral 84, including a pair ofsensors and 86. The sensors 85 and 86 are identical to each other. Thesesensors have base electrodes 85.1 and 86.1, and ring electrodes 85.2 and86.2 which define inwardly facing cylindrical annular electrode faces85.3, 86.3 inperpendicular and spaced relation with the flat electrodefaces 85.4, 86.4 of the base electrodes. The base and ring electrodes ofsensors 85 and 86 are constructed and arranged substantially identicallywith the electrodes of the sensor 50 described and illustrated inconnection with FIG. 1, with a few exceptions as will be pointed out.

The base electrodes 85.1, 86.1 have conically shaped side surfaces 85.5which converge in a direction away from the ring electrodes and convergetoward the rigid mounting panel 87 which is common to both the sensors85, 86. In this dual unit 84, the base and ring electrodes of bothsensors 85 and 86 are suspended in depending relation from the mountingpanel 87. The cylindrical mounting projections or walls 85.6, 86.6 ofthe ring electrodes, bear against the lowersurface or reference plane87.1 of the mounting panel 87 and are clamped thereagainst by screws 88which are screwed into tapped apertures in the cylindrical wall portions85.6, 86.6 of the ring electrodes. I

Screws 88 also serve to clamp the sensing unit 84 to a rigid mountingbracket 89 which hasenlarged open ings 89.1 adjacent each of the sensors85, 86.

The mounting stems 85.7, 86.7 of the base electrodes bear against thelower surface or reference plane 87.1 of the rigid mounting panel 87 andare clamped thereagainst by the reduced threaded ends of mounting studs90 which are screwed into tapped apertures extending axially of themounting stems. v

The insulators 85.8, 86.8 which separate the base and ring electrodesand' cooperate therewith in defining a chamber to contain the liquiddielectric material to be measured, have a conical exterior shape to bespaced from substantial portions of the cylindrical mounting projectionsor walls 85.6, 86.6 whereby to define annular air spaces for the purposeof reducing stray capacity because of the low dielectric constant (1.0)of air. The insulators 85.8, 86.8 are-molded integrally of the ringelectrodes 85.2, 86.2 which are apertured at 85.2 and 86.2 to receivesmall plugs of the insulator and thereby affixedly position theinsulators with respect to the ring electrode.

A single plastic molding 91 defines housing 91.1 and 91.2 which enclosesensors '86 and 85 respectively. Each of the housings has a longitudinalflow passage 91.3 extending therethrough in a direction transversely oftheaxes of the base and ring electrodes. The flow passages. 91.3 in theplastic molded housings have threaded nipples or pipe fittings 92threaded into the housings for flow communication and to facilitateconnection to pipe fittings. The housings are provided with enlargedtransverse bores 91.4 extending transversely of the flow passages 91.3and oriented in concentric alignment with the respective sensors 85, 86so as to provide open flow communication between the flow passages 91.3and the sensors 85, 86.

Theplastic molding 91 defining the housings which are sealed to sensors85, 86, is affixed to the mounting bracket 89 and panel 87 by clampingbolts 93.

The plastic molding has cylindrical openings to receive the ringelectrodes 85.2, 86.2 and .to snugly seal against the exterior or lowerflat surfaces thereof and the peripheral cylindricalwall surfaces ofthemounting projections 85.6, 86.6. As a result, the sensors 85, 86 areexposed only to the dielectric material which is flowing in the passageof the particular housing 91.1, 91.2.

It is intended that the sensor unit 84 be employed for the purpose ofcontinuallysensing and monitoring the dielectric characteristics of alow dielectric liquid material in use, such as the lubricating oil of aninternal combustion engine. The flow passage through housing 91.2 willbe connected at the fittings 92 to the flow line for the recirculatinglubricating oil of the engine so that oil at engine temperature will becontinuously supplied and constantly changed at the sensor 85. Aquantity of low dielectricmaterial, and in this case lubricating oil, innew and unused condition is supplied into the flow passage of housing91.1 so as to be constantlyexposed to the sensor 86 and to fill thechamber adjacent the base and ring electrodes thereof. Closure caps 92.1are applied to conf ne the quantity of new andunused liquid lowdielectric material at the sensor 86 to provide a norm to whichreference will be made in comparing the characteristics of recirculatingoil being sensed and monitored by the sensor 85. Because the sensor 86is mounted in close proximity with the sensor 85, and

from the same mounting bracket, the sample of oil in the sensor 86 willbe substantially the same temperature as the recirculating sample oflubricating oil being continuously exposed to sensor 85.

.The mounting post or studs also carry a fiberglass circuit board 94,which will carry substantial portions of the capacitance bridge circuitillustrated in FIG. 11. The fiberglass panel 94 will carry a pairofvariable coils 68 and 71 with rotary adjustment pins to vary theinductance thereof. The circuit board and the components thereon andalso the coils 68, 71 are enclosed within and confined by a housingwhich is clamped downwardly against the mounting bracket 89 and sealedthereto by gasket 95.1.

With respect to the bridge circuit illustrated in FIG. 7 which is a partof the sensor unit 84, substantially the entire bridge circuit is thesame as illustrated in FIG. 2, and the same numerals on all of theidentical components and circuit points are repeated in FIG. 7 to showthe very substantial similarity. The principal difference 'in thecircuit of FIG. 11 is the addition of a fail-safe circuit to indicatethat the circuit is operating properly. In this regard, a single poledouble throw switch 96.5 is inserted between the meter 78 and the wiperof potentiometer 81. The second pole of the switch 96.5 is connecteddirectly to a current limiting resistor 96.4 of approximately 27,000ohms. The resistor 96.4 is connected in series with a diode 96.3,identical in characteristics to diodes 73, 74, and the diode 96.3rectifies the negative half cycle of AC voltage appearing at point 64 atthe oscillator frequency. The diode 96.3 is connected in series with acoil 96.1 which is identical to coil 67, and coil 96.1 is connecteddirectly to point 64 of the circuit. The midpoint between the diode 96.3and coil 96.1 is connected to ground 65 through a capacitor 96.3 and isidentical to capacitors 79, 80 as to block DC current to ground, buteffectively pass AC voltages of the oscillator frequency to ground 65.The combined-purpose of coil 96.1, diode 96.3 and resistor 96.4 is toprovide a fail-safe indication on meter 78 when switch 96.5 is placed intest position. Once the circuit is calibrated, the AC voltage appearingacross circuit point 64 to ground 65 remains nearly constant and servesas an indication the circuit functioning properly.

Also in this circuit, the supply of voltage is provided at terminal60.1, and, as in the circuit of FIG. 2, a voltage of 5 to 10 volts DC issupplied to the oscillator 61. The variable resistor 96 between thepower supply terminal 60.1 and the oscillator 61 will vary the inputvoltage supplied to the oscillator. Variable resistor 96 is a ohm wirewound resistor.

In this circuit of FIG. 7, the sensor 85 is exposed to the flowingliquid dielectric material, as indicated by the arrows. This sensor 85replaces the sensor 50 of FIG. 2. The variable capacitor 72 of FIG. 2 isreplaced by the other sensor 86 of unit 84 which contains a small purestate sample of the unused dielectric liquid which is being recirculatedand monitored by sensor 85. The unit 84 provides a direct comparison ofthe dielectric constant of the liquid dielectric material being sensedand monitored. It may be desirable that the meter and test switch 96.5be vlocated at a position remote from the .unit84, and in the case of astationary'combustion engine as used in power generating plants, themeter may be several miles away.

As hereinbefore explained, neither of the sensors 85 or 86 will vary theeffective capacity thereof by reason of a change of temperature. The useof the second sensor 86 provides a constant reference to the normagainst which the comparison is made. Thevariable inductance coils68,'71are exposed to identical conditions within the cover 9 'of the unit andafter they are originally adjusted to balance the system will remain inbalance with each other.

In the use of the sensors as described herein, there is no requirementas to the orientation of the sensors with respect to the vertical. Thesensors must only be completely exposed over their entire electrodesurface areas to the dielectric material, the dielectric characteristicsof which are being measured. In the sensing of the dielectriccharacteristics of a high dielectric mate rial, the electrode face ofone of the electrodes, or of both of the electrodes may be very thinlycoated with an insulating material, preferably at a thickness of 0.005to 0.0l0 inches.

It will be seen that we have provided a new and improved sensor fordetermining the dielectric characteristics of a fluid dielectricmaterial with a high degree of accuracy so that conclusions can be drawnas to the nature of the dielectric material and any contaminants thatmaybe contained therein. The sensoris characterized by a base electrodewith a flat electrode face which forms one end of a container or chamberwherein the fluid dielectric material is confined. A ring electrode withan annular electrode face is disposed normal to the flat electrode faceof the base electrode, and is spaced from the peripheral edge of thebase electrode face. Both the base electrode and the ring electrode aremounted on and secured to a ridgemounting at a common reference-plane;the diameters of the flat and cylindrical electrodefaces are identical,and the width of the wafer-shaped portion of the ring electrode is thesame as half the diameter of the flat electrodeface. In certain forms ofthe sensor, the base electrode may be adjustable slightly with respectto the ring electrode for initial tuning. Both the flat electrode faceand the annular electrode face are oriented parallel to the referenceplane of the rigid mounting. An insulator of stable low dielectricmaterial which is substantially insensitive to temperature changes andis of such strength as to yield to the substantial strength of thesimilar metal in the base and ring electrodes, is provided between thering and base electrodes to confine the liquid dielectric material inthe chamber or container and also to minimize stray or shunt capacitybetween the base electrode and the ring electrode. A capacitance bridgewhich is highly sensitive to change in capacity of the sensor is usedfor detecting changes in the capacity produced by variances in thedielectric characteristics of the fluid dielectric material ln acontinuous monitoring form for monitoring the characteristics of asupply of liquid during use, a second sensor containing a quantity ofthe same nature of liquid, but in a pure and unused state, is utilizedas a reference or norm against which the comparison is made to thecharacteristics of the material being used and recirculated.

What is claimed is:

l. A sensor for determining the dielectric characteristics of a fluidmedium, comprising:

means defining a rigid mounting lying in a plane,

a rigid metallic baseelectrode having a generally flat electrode facelying substantially parallel to said plane and also having an annularedge at the periphery of the face, said base electrode being se cured tosaid rigid mounting,

a rigid ring electrode of the same material as the base electrode andspaced from the base electrode, the ring electrode having an inwardlyfacing annular electrode face conforming to the size and shape I andorientation of the annular edge of the base electrode face, said annularelectrode face lying substantially perpendicular to the flat face of thebase electrode and having an annular end edge adjacent the flatelectrode face and uniformly spaced from the flat electrode face aroundthe periphery thereof, said ring electrode having a width, in adirection outwardly from the annular electrode face, equal to thedistance from the center of the flat electrode face to the annular edgethereof, said ring electrode also being secured to said rigid mounting,and

fluid sealing insulator means disposed between the base and ringelectrodes and closing the space between the annular edges of theelectrodes, the insulator means being formed of a solid insulating lowdielectric material with characteristics of minimum change of dielectricconstant in response to changes in temperature, and said insulatingmaterial having strength characteristics considerably weaker than thematerial of the electrodes to allow limited relative movement betweenthe electrodes during temperature induced expansion and contraction, andsaid insulating means cooperating with the electrodes in defining anopen ended fluid medium-confining chamber with the base electrodeforming one end of the chamber and the ring electrode forming a portionof the peripheral wall of the chamber.

2. The sensor according to claim 1 and the area of the annular electrodeface being substantially equal to the area of the flat electrode face.

3. The sensor according to claim 2 and the area of the flat electrodeface being at least as large as the area of the annular electrode face.

4. The sensor according to claim 2 and the flat electrode face of thebase electrode being circular, and said annular electrode face alsobeing circular.

5. The sensor according to claim 2 and the base electrode having amounting stem extending toward and secured to the rigid mounting andhaving a thickness significantly less than the distance across the flatelectrode face.

6. The sensor according to claim 2 and the ring electrode beingsubstantially flat and lying substantially parallel with the flatelectrode face of the base electrode.

7. The sensor according to claim 6 and said ring electrode having amounting projection at the outer periphery of the electrode andextending to said rigid mounting.

8. The sensor according to claim 7 and said mounting projectioncomprising a cylindrical wall formed integrally of and in one piece withthe flat portion of the ring electrode.

9. The sensor according to claim 7 and the base electrode having amounting stem extending to said rigid mounting and having a thicknesssignificantly less than the distance across the flat electrode face, thespace between the stem of the base electrode and the mounting projectionof the ring electrode being filled with insulating medium including aportion of said fluid sealing insulator means.

10. The sensor according to claim 1 wherein the sensor is oriented toposition the open-ended chamber in upright position with the baseelectrode forming the bottom end of the chamber, and insulating meansdefining a peripheral wall above the ring electrode to extend thechamber upwardly and facilitate confining an increased depth of thefluid medium.

11. The sensor according to claim 1 and a housing having a flow passagetherethrough and through which said fluid medium may flow, said housingembracing the sensor and providing open flow communication between saidpassage and said open-ended fluid medium confining chamber for renewingthe fluid medium in the chamber with the fluid medium from the passage.

12. The sensor according to claim 1 and the base electrode having abeveled side wall adjacent said annular edge, said beveled side wallfacing obliquely away from the ring electrode.

13. The sensor according to claim 1 and said base electrode beingmovable toward and away from the ring electrode to vary the spacingtherebetween and to vary the capacitance of the sensor.

14. The sensor according to claim 13 and said base electrode having adisc portion defining said flat electrode face and also having athreaded stem extending to and being threaded into the rigid mounting tofacilitate adjustment of the base electrode relative to the ringelectrode.

15. A sensor for determining the dielectric characteristics of a fluidmedium, comprising:

means defining a rigid mounting lying in a plane,

a rigid metallic base electrode having a generally flat electrode facelying substantially parallel to said plane and also having an annularedge at the periphery of .the face, said base electrode engaging andbeing secured to said rigid mounting at the plane,

a rigid ring electrode of the same material as the base electrode andspaced from the base electrode, the ring electrode having an inwardlyfacing annular electrode face conforming to the size and shape andorientation of the annular electrode of the base electrode face, saidannular electrode face lying substantially perpendicular to the flatface of the base electrode and said annular electrode face beinguniformly spaced from the flat electrode around the periphery thereof;said ring electrode having a thickness in a direction across the annularelectrode face, and also having a width in a direction outwardly fromthe annular face of such magnitudes as to cause the area of the annularelectrode face to change corresponding to the change of the baseelectrode face with change in temperature and proportionately to thechange in spacing between the electrode faces of the base and ringelectrodes as the electrodes extend from or contract toward thereference plane of the rigid mounting in response to such change intemperature, and fluid sealing insulator means disposed between the baseand ring electrodes and closing thespace between the peripheries of theelectrodes, the insulator means being formed of a solid insulating lowdielectric material with characteristics of minimum change of dielectricconstant in response to changes in temperature, and said insulatingmaterial having strength characteristics considerably weaker than thematerial of the electrodes to allow limited relative movement betweenthe electrodes during temperature induced expansion and contraction, andsaid insulating means cooperating with the electrodes in defining anopen ended fluid medium-confining chamber with the base electrodeforming one end of the chamber and the ring electrode forming a portionof the peripheral wall of the chamber. 16. The sensor according to claim15 and'said annular electrode face having an area at least as small asthe area of the flat electrode face of the base electrode. a

1. A sensor for determining the dielectric characteristics of a fluidmedium, comprising: means defining a rigid mounting lying in a plane, arigid metallic base electrode having a generally flat electrode facelying substantially parallel to said plane and also having an annularedge at the periphery of the face, said base electrode being secured tosaid rigid mounting, a rigid ring electrode of the same material as thebase electrode and spaced from the base electrode, the ring electrodehaving an inwardly facing annular electrode face conforming to the sizeand shape and orientation of the annular edge of the base electrodeface, said annular electrode face lying substantially perpendicular tothe flat face of the base electrode and having an annular end edgeadjacent the flat electrode face and uniformly spaced from the flatelectrode face around the periphery thereof, said ring electrode havinga width, in a direction outwardly from the annular electrode face, equalto the distance from the center of the flat electrode face to theannular edge thereof, said ring electrode also being secured to saidrigid mounting, and fluid sealing insulator means disposed between thebase and ring electrodes and closing the space between the annular edgesof the electrodes, the insulator means being formed of a solidinsulating low dielectric material with characteristics of minimumchange of dielectric constant in response to changes in temperature, andsaid insulating material having strength characteristics considerablyweaker than the material of the electrodes to allow limited relativemovement between the electrodes during temperature induced expansion andcontraction, and said insulating means cooperating with the electrodesin defining an open ended fluid medium-confining chamber with the baseelectrode forming one end of the chamber and the ring electrode forminga portion of the peripheral wall of the chamber.
 2. The sensor accordingto claim 1 and the area of the annular electrode face beingsubstantially equal to the area of the flat electrode face.
 3. Thesensor according to claim 2 and the area of the flat electrode facebeing at least as large as the area of the annular electrode face. 4.The sensor according to claim 2 and the flat electrode face of the baseelectrode being circular, and said annular electrode face also beingcircular.
 5. The sensor according to claim 2 and the base electrodehaving a mounting stem extending toward and secured to the rigidmounting and having a thickness significantly less than the distanceacross the flat electrode face.
 6. The sensor according to claim 2 andthe ring electrode being substantially flat and lying substantiallyparallel with the flat electrode face of the base electrode.
 7. Thesensor according to claim 6 and said ring electrode having a mountingprojection at the outer periphery of the electrode and extending to saidrigid mounting.
 8. The sensor according to claim 7 and said mountingprojection comprising a cylindrical wall formed integrally of and in onepiece with the flat portion of the ring electrode.
 9. The sensoraccording to claim 7 and the base electrode having a mounting stemextending to said rigid mounting and having a thickness significantlyless than the distance across the flat electrode face, the space betweenthe stem of the base electrode and the mounting projection of the ringelectrode being filled with insulating medium including a portion ofsaid fluid sealing insulator means.
 10. The sensor according to claim 1wherein the sensor is oriented to position the open-ended chamber inupright position with the base electrode forming the bottom end of thechamber, and insulating means defining a peripheral wall above the ringelectrode to extend the chamber upwardly and facilitate confining anincreased depth of the fluid medium.
 11. The sensor according to claim 1and a housing having a flow passage therethrough and through which saidfluid medium may flow, said housing embracing the sensor and providingopen flow communication between said passage and said open-ended fluidmedium confining chamber for renewing the fluid medium in the chamberwith the fluid medium from the passage.
 12. The sensor according toclaim 1 and the base electrode having a beveled side wall adjacent saidannular edge, said beveled side wall facing obliquely away from the ringelectrode.
 13. The sensor according to claim 1 and said base electrodebeing movable toward and away from the ring electrode to vary thespacing therebetween and to vary the capacitance of the sensor.
 14. Thesensor according to claim 13 and said base electrode having a discportion defining said flat electrode face and also having a threadedstem extending to and being threaded into the rigid mounting tofacilitate adjustment of the base electrode relative to the ringelectrode.
 15. A sensor for determining the dielectric characteristicsof a fluid medium, comprising: means defining a rigid mounting lying ina plane, a rigid metallic base electrode having a generally flatelectrode face lying substantially parallel to said plane and alsohaving an annular edge at the periphery of the face, said base electrodeengaging and being secured to said rigid mounting at the plane, a rigidring electrode of the same material as the base electrode and spacedfrom the base electrode, the ring electrode having an inwardly facingannular electrode face conforming to the size and shape and orientationof the annular electrode of the base electrode face, said annularelectrode face lying substantially perpendicular to the flat face of thebase electrode and said annular electrode face being uniformly spacedfrom the flat electrode around the periphery thereof; said ringelectrode having a thickness in a direction across the annular electrodeface, and also having a width in a direction outwardly from the annularface of such magnitudes as to cause the area of the annular electrodeface to change corresponding to the change of the base electrode facewith change in temperature and proportionately to the change in spacingbetween the electrode faces of the base and ring electrodes as theelectrodes extend from or contract toward the reference plane of therigid mounting in response to such change in temperature, and fluidsealing insulator means disposed between the base and ring electrodesand closing thespace between the peripheries of the electrodes, theinsulator means being formed of a solid insulating low dielectricmaterial with characteristics of minimum change of dielectric constantin response to changes in temperature, and said insulating materialhaving strength characteristics considerably weaker than the material ofthe electrodes to allow limited relative movement between the electrodesduring temperature induced expansion and contraction, and saidinsulating means cooperating with the electrodes in defining an openended fluid medium-confining chamber with the base electrode forming oneend of the chamber and the ring electrode forming a portion of theperipheral wall of the chamber.
 16. The sensor according to claim 15 andsaid annular electrode face having an area at least as small as the areaof the flat electrode face of the base electrode.